Activated silica sols have been used for many years to coagulate suspended particles in water. The silica sols envelop the particles and settle out of solution, thereby clarifying the water. Activated silica sols can be used alone, or, as is more usual, together with other conventional coagulants, such as alum (Al.sub.2 (SO.sub.3).sub.3.14H.sub.2 O), lime or iron salts, that remove both suspended particles and color bodies from water. Activated silica sols are also used in paper making for retention, drainage and strength improvements. This industry employs the term "colloidal silica" for activated silica sols.
Sodium silicate is made by reacting SiO.sub.2 with Na.sub.2 O so that the resultant sodium silicate contains a SiO.sub.2 :Na.sub.2 O weight ratio between 3.22-3.25:1. The silica sols are activated by adding chlorine, alum or other material such as sodium bicarbonate, sulfuric acid or ammonium sulfate, to the diluted silica sol. More recently, sodium aluminate has also been reacted with sodium silicate for producing an activated silica sol.
The combined suspended particle/silica sol is a fast acting flocculents. Desirable flocculent must settle rapidly in water by forming large, heavy particles. The supernatant clarified water reduces the costs of filtering the water by increasing flow through rates and reducing both the frequency and length of backflushing. Also, less of conventional materials used to remove suspended particles and color bodies from water, such as alum and lime, are required. The activated silica sol is used to precipitate solids that are reclaimable as well, such as fibers, fillers and the like from paper manufacturing waste waters, thus reducing the overall costs of paper manufacture.
However, activated silica sols must be made batchwise, on site, just prior to use because they gel or solidify in a short period of time, e.g., about two hours or less, when they become unusable. This requires blending systems, dosage systems and handling equipment also to be located on site so that the activated silica sol can be used before it solidifies or gels. Great care must be taken that the activated silica sol does not solidify in pipes, joints and the like.
Thus it would be highly desirable to produce an activated silica sol that is stable, and can be premixed and stored for long periods of time without gelling or solidifying. Such a stable activated silica sol could also be used "on line", added continuously during generation of waste waters to remove suspended materials and color bodies continuously from wash or waste water streams during manufacturing processes.